Spider silks are an ideal system for exploiting the relationship between protein design and function. While silk production has evolved multiple times within arthropods, silk use is most highly developed in spiders. Spiders are unique both in their dependence on and ability to spin an array of silk proteins throughout their lifetimes. Each type of silk is secreted and stored by a different type of abdominal gland until extruded by tiny spigots on the spinnerets. These proteins are used singly or in combinations for draglines, retreats, egg sacs, or prey-catching snares. Given these specialized ecological roles, individual silks appear to have mechanical properties that correspond to their individual functions.
Most molecular and structural investigations on spider silks have focused on dragline silk and its extreme toughness (e.g. Xu & Lewis, Proc. Natl. Acad. Sci., USA 87, 7120-7124, 1990; Hinman & Lewis, J. Biol. Chem. 267, 19320-19324, 1992; Thiel et al., Biopolymers 34, 1089-1097, 1994; Simmons et al., Science 271, 84-87, 1996; Kummerlen et al., Macromol. 29, 2920-2928, 1996; and Osaki, Nature 384, 419, 1996). Dragline silk, often referred to as major ampullate silk because it is produced in the major ampullate glands, has a tensile strength (5.times.10.sup.9 Nm.sup.-2) similar to Kevlar (4.times.10.sup.9 Nm.sup.-2) (Gosline et al., Endeavour 10, 37-43, 1986; Stauffer et al., J. Arachnol. 22, 5-11, 1994). In addition to this exceptional strength, dragline silk also exhibits .about.35% elasticity (Gosline et al., Endeavour 10, 37-43, 1986). Thus a structure/function understanding of dragline silk must account for both strength and elasticity.
Silk strength is widely attributed to crystalline .beta.-sheet structures. Such protein domains are found in both lepidopteran silks (e.g. Bombyx mori, Mita et al., J. Mol. Evol. 38, 583-592, 1994) and spider silks (Xu & Lewis, Proc. Natl. Acad. Sci., USA 87, 7120-7124, 1990; Hinman & Lewis, J. Biol. Chem. 267, 19320-19324, 1992; Gosline et al., Endeavour 10, 37-43, 1986). In contrast, elasticity is generally thought to involve amorphous regions (Wainwright et al., Mechanical design in organisms, Princeton University Press, Princeton, 1982). More precise characterization of these amorphous components can be revealed by molecular sequence data.
Based on the protein sequences of major ampullate silk proteins, a .beta.-turn structure was suggested to be the likely mechanism of elasticity (Hinman & Lewis, J. Biol. Chem. 267, 19320-19324, 1992). However, assessing this proposition was problematic because dragline silk is a hybrid of at least two distinctive proteins which must impart both strength and moderate elasticity.
To elucidate the basis of silk elasticity, the present inventors have cloned a component of the stretchiest of silks; the capture spiral of an orb-web. The capture thread has a lower tensile strength (1.times.10.sup.9 Nm.sup.-2) but several times the elasticity (&gt;200%) of dragline silk (Vollrath & Edmonds, Nature 340, 305-307, 1989; Kohler & Vollrath, J. Exp. Zool. 271, 1-17, 1995). The capture spiral is formed from both flagelliform and aggregate gland silks. However, the present inventors focused on flagelliform silk because it is the core fiber of the spiral while aggregate silk provides a non-fibrous, aqueous coating. Thus, while aggregate silk is an integral part of the elastic capture spiral, it is flagelliform silk that must actually stretch. The present inventors report the cloning of substantial cDNA for this silk protein.